Role of Phosphatidylinositol(4,5)bisphosphate Organization in Membrane Transport by the Unc104 Kinesin Motor

Unc104 (KIF1A) kinesin transports membrane vesicles along microtubules in lower and higher eukaryotes. Using an in vitro motility assay, we show that Unc104 uses a lipid binding pleckstrin homology (PH) domain to dock onto membrane cargo. Through its PH domain, Unc104 can transport phosphatidylinositol(4,5)bisphosphate (PtdIns(4,5)P 2)-containing liposomes with similar properties to native vesicles. Interestingly, liposome movement by monomeric Unc104 motors shows a very steep dependence on PtdIns(4,5)P 2 concentration (Hill coefficient of ∼20), even though liposome binding is noncooperative. This switch-like transition for movement can be shifted to lower PtdIns(4,5)P 2 concentrations by the addition of cholesterol/sphingomyelin or GM1 ganglioside/cholera toxin, conditions that produce raft-like behavior of Unc104 bound to lipid bilayers. These studies suggest that clustering of Unc104 in PtdIns(4,5)P 2-containing rafts provides a trigger for membrane transport.